1. Field of the Invention
The invention pertains to a thermogenerator with several thermoelectric pairs connected electrically to each other arranged between the hot side of the thermogenerator, which accepts the incoming heat, and a cold side of the thermogenerator arranged a distance away from the thermogenerator.
2. Related Art
A thermocouple operated as a thermogenerator produces electrical voltage on the basis of the Seebeck effect. In most cases, a relatively large number of pairs of thermorgenerator elements are connected together to form a thermocouple. The thermogenerator can comprise one or more thermocouples, which are connected electrically in series and/or in parallel. The thermoelectric voltage generated in the thermocouples is temperature-dependent and is in the range of a few microvolts. A few alloys have become widely accepted as thermoelectric pairs because of their properties at certain temperatures, and therefore a spectrum of thermoelectric material combinations (thermoelectric pairs) extending over a temperature range from −270° C. to 2600° C. has been developed. This spectrum is covered and defined by standards. The currently valid international standard for thermocouples is IEC 584-1, the counterpart to which in German-speaking countries is DIN EN 60584, Part 1. This standard defines 10 different thermoelectric material combination in terms of their properties:
Type/Code LetterAlloyKnickel-chromium/nickel-aluminumTcopper/copper-nickelJiron/copper-nickelNnickel-chromium-silicon/nickel-siliconEnickel-chromium/copper-nickelRplatinum-13% rhodium/platinumSplatinum-10% rhodium/platinumBplatinum-30% rhodium/platinum
Another standard also used in Germany is DIN 43710, which defines thermoelectric types U and L. This standard is no longer valid.
Ucopper/copper-nickelLiron/copper-nickel
In addition to the standardized thermoelectric pairs, there are also other combinations with special properties. Examples include the tungsten/tungsten-rhenium combination with possible temperature ranges up to 2600° C.
As conductive materials for the thermoelectric pairs of thermocouples, p-doped and n-doped semiconductor materials, usually bismuth-tellurite Bi2Te3 in particular, can also be considered. In addition, the p-doped and n-doped compounds listed in the following Tables 1.1 and 1.2 can be used:
TABLE 1.1The p-type compounds with the best thermoelectric properties.T (K)Compound, p-typeZ (1/K)225CsBi4Te6:SbI3 (0.05%)3.5-10−3300(Sb2Te3)72(Bi2Te3)25(S2Se3)33.4-10−3500TI9BiTe62.3-10−3700GeTe1-x(AgSbTe2)x3.0-10−31200Si0.85Ge0.15:B6.7-10−4
TABLE 1.2The n-type compounds with the best thermoelectric properties.T (K)Compound, n-typeZ (1/K)80Bi0.85Sb0.15   6.5-10−3300((Sb2Te3)5Bi2Te3)90(Sb2Se3)5   3.2-10−3450Bi2Te2.7Se0.3   2.8-10−3800Pb0.75Sn0.25Se>1.25-10−31200Si0.85Ge0.15:P   8.3-10−4
A thermocouple operated as a thermogenerator normally comprises two thin heat-conducting plates, especially ceramic plates, between which alternating small squares of different conductive material, especially semiconductor material, are brazed in place. In each case, two different squares are connected to each other in such a way that they produce a series circuit. One of the two plates accepts the incoming heat flow (in the following also called the “hot side” of the thermocouple), whereas the other plate releases the outgoing heat flow (in the following also called the “cold side” of the thermocouple).
In addition to conventional thermocouples arranged between plates, it is also possible to use thin-film thermocouples in particular, such as those known by way of example from DE 101 22 679 A1. Thin-film thermocouples also have a hot side and a cold side.
The known thermoelectric generators are able to convert heat directly into electrical energy. The efficiency can be increased significantly in comparison with conventional thermocouples by using semiconductor materials in place of metals. Nevertheless, the thermogenerators available today have only a relatively low degree of efficiency; it is only a fraction (approximately 17%) of the Carnot efficiency.
A device for recovering electrical energy from solar radiation, furthermore, is known from DE 195 37 121 A1, wherein, in contrast to conventional photovoltaic systems, the solar radiation is not used directly. Instead, the radiation is converted into heat, which is then converted at a different location into electrical energy by Peltier elements in conjunction with a heat sink. The device comprises a collector for transferring the energy of the solar radiation to a first heat-transfer medium, and, in a location remote from the collector, a heat exchanger for transferring the heat from the first heat-transfer medium to the hot side of the Peltier elements, the cold side of which is connected to a heat sink.
A thermal solar collector is known from U.S. Pat. No. 6,857,425 B2, which comprises a collector plate, a first plate facing away from the solar radiation, and a support plate arranged between the other two plates and insulated on both sides; the support plate forms an upper channel and a lower channel for a heat-transfer medium circulated between the upper and lower channels. On one of the end surfaces of both the upper and lower channels there is a heat-conductive device, along which the heat-transfer medium flows. This device conducts the heat absorbed in the upper channel to a thermopile unit, which generates electricity from the heat. A heat exchanger is connected to the thermopile unit to carry heat away from the unit and thus to increase its efficiency. The heat that is carried away can be used, for example, to heat a building.